Tuberculosis (TB) continues to be a major public health threat around the world. The estimate that more lives were lost in 2009 due to TB than in any year in history and more people died in 2014 from tuberculosis than from HIV/AIDS is alarming. An increasing number of cases reporting infection with multi-(MDR) and extensively drug-resistant (XDR) strains of M. tuberculosis has diminished our capability to respond effectively against this threat. A recent study reporting high mortality rates of patients co-infected with HIV and XDR-TB illustrates the need for new drugs to treat TB. It is speculated that poor patient compliance to treatment regimens, as the current therapy requires a combination of drugs to be taken daily for 6 months or more, is a major reason for emergence of drug resistance in TB. While >99% of M. tuberculosis bacilli are killed within 2 weeks of therapy, it takes the remainder of the therapy to effectively kill the surviving population. These bacilli, broadly termed “persisters”, are able to transiently tolerate drugs. The phenomenon of persistence is poorly understood. In vitro models designed to mimic the physiology of persisters are based on exposure to nitric oxide and depletion of oxygen and nutrients as these conditions are thought to prevail in a persisting infection in vivo.
A higher percentage of bacilli are able to survive exposure to drugs at stationary phase compared to exponential phase growth. The bacterial cell wall, as an interface between the pathogen and the host, regulates diffusion, influx and efflux of drugs and metabolites. Integrity and permeability of this interface is highly significant to effective targeting of M. tuberculosis with drugs. Little is known about changes in the cell wall during chronic phase of infection and whether it regulates persistence of M. tuberculosis in the host. Until recently, it was thought that D,D-transpeptidases (commonly known as penicillin binding proteins), which catalyze the synthesis of cross-linked peptide bonds between the 4th amino acid of one stem peptide and 3rd amino acid of another thereby forming 4→3 linkages, was the only class of enzymes involved in the final step of peptidoglycan (PG) biosynthesis. The mechanism by which M. tuberculosis maintains 3→3 cross-linkages in the peptidoglycan layer has recently been identified by the discovery of an M. tuberculosis, LdtMt2, encoding for an L,D-transpepetidase and identification of its role as a catalyst for the formation of non-classical 3→3 cross-linkages in the peptidoglycan layer. Inactivation of the gene encoding LdtMt2 protein results in altered colony morphology, attenuation in growth, loss of virulence, and increased susceptibility to β-lactams and β-lactamase inhibitors in vitro and during the chronic phase of tuberculosis infection as demonstrated in the mouse model of the disease. Non-classical 3→3 cross linkages predominate the transpeptide network of the peptidoglycan layer of non-replicating M. tuberculosis. The peptidoglycan network is a dynamic structure that is cross-linked by both 4→3 and 3→3 transpeptide linkages. Both L, D and D,D-transpeptidases are involved in the maintenance and remodeling of the peptidoglycan network in M. tuberculosis. New inhibitors to the recently identified L,D-transpeptidase must be discovered to develop new antibacterial agents enabling growth inhibition of bacteria strains resistant to conventional drugs. Of broader significance is the emerging fact that 3→3 linkages and l,d-transpeptidases are present in a wide range of bacteria such as E. coli, Pseudomonas spp., K. pneumoniae, Streptomyces spp., C. difficile, Actinomycetales spp., E. faecium, E. faecalis, A. baumannii and E. cloacae. 